IPC-TM-650 EN 2022 试验方法--.pdf - 第550页
the test fixture. The accuracy of the measurement relies highly on the quality of th e physical calibration stand ards, espe cially for SOLT type of cal ibration standards, where the parasitics of the SO LT calibration s…
1 Scope and Purpose
1.1 Scope
This document describes the frequency domain
test methods to accurately determine the amount of signal
propagation loss and delay for electrical printed boards, to
meet the demand of high speed applications nowadays. As
the data rate of high speed IO continues to increase (e.g., 10
Gbps and above), production testing and development testing
require more precise and accurate high frequency methods.
(Existing IPC-TM-650 Test Methods such as Method
2.5.5.12A are not adequate). Additionally, previous IPC test
methods do not encompass traditional industry methods
using VNA, such as thru-reflect-line (TRL), and recent devel-
opments of 2X-Thru test methods, etc. This test method is
defined to close the gaps.
The scope of this test method includes:
• Calibration and/or de-embedding techniques
• Probing/test fixture choices that impact measurement
quality
• Coupon Design
• Test sample pre-conditioning
• Environmental impact, etc.
1.2 Purpose
1.2.1 The importance of Setting up Correct Reference
Plane for Printed Board Characterization
The impor-
tance of setting up a correct reference plane in a typical inter-
connect measurement setup is illustrated in Figure 1-1. The
vector network analyzer (VNA) has been the de-facto standard
for accurate passive interconnect characterization including
the printed circuit board, connector, cables, etc. Making high
quality VNA measurement is straight-forward with standard
coaxial connectors and precision SOLT (short, open, load,
through) calibration kits. However, test fixtures are usually
required to connect the standard coaxial connectors to the
non-coaxial device under test (DUT). SOLT calibration can
readily move the reference plane to Ref plane A and Ref plane
A’ in the figure, while the intended DUT is the printed board
conductor only (between Ref plane B and Ref plane B’). The
test fixtures (between A and B, A’ and B’) need to be charac-
terized and then de-embedded to recover the insertion loss of
DUT.
Microwave probes are often used to probe interconnect struc-
tures for quick measurement, as shown Figure 1-2. A similar
calibration or de-embedding procedure is needed to move the
reference plane to the target location (Ref plane B and B’
shown in the figure). Note that sometimes, an SOLT calibra-
tion procedure can be carried out using calibration substrates
provided by probe vendor, to move the reference plane to the
probe tip, but it does not move the reference plane to the tar-
get location and additional de-embedding procedure is still
needed.
In a general calibration/de-embedding process, specialized
calibration standards with known electrical properties are
inserted at the end of the test fixture, and a calibration pro-
cess is performed to move the reference plane to the end of
IPC-25514-1-1
IPC-25514-1-2
3000 Lakeside Drive, Suite 105N
Bannockburn, IL 60015-1249
IPC-TM-650
TEST METHODS MANUAL
Number
2.5.5.14
Subject
Measuring High Frequency Signal Loss and
Propagation on Printed Boards with Frequency
Domain Methods
Date
02/2021
Revision
Originating Task Group
High Frequency Signal Loss Test Methods Task
Group (D-24D)
C/PC@
BUILD
ELECTRONICS
Ref
plane
A
Ref
plane
A’
Ref
plane
B
Ref
plane
B'
Figure
1-1
Reference
Planes
in
Printed
Board
Insertion
Loss
Characterization
Ref
plane
B
Ref
plane
B*
Figure
1-2
Reference
Planes
in
Printed
Board
Insertion
Loss
Characterization
with
Microwave
Probe
Material
/n
this
Test
Methods
Manual
was
voluntarily
established
by
Technical
Committees
of
I
PC.
This
material
/s
advisory
only
and
"s
use
or
adaptation
，
s
entirely
voluntary.
IPC
disclaims
all
liability
of
any
kind
as
to
the
use,
application,
or
adaptation
of
this
material.
Users
are
also
wholly
responsible
for
protecting
themselves
against
all
claims
or
liabilities
for
patent
infringement.
Equipment
referenced
/s
for
the
convenience
of
the
user
and
does
not
imply
endorsement
by
IPC.
Page
1
of
11
the test fixture. The accuracy of the measurement relies highly
on the quality of the physical calibration standards, especially
for SOLT type of calibration standards, where the parasitics of
the SOLT calibration standard must be known a priori. How-
ever, for printed board structures, it is not feasible to build an
accurate broadband SOLT structure right after the test fixture.
Hence the on-board SOLT calibration process usually does
not work well above a few GHz.
There are existing calibration/de-embedding methods in the
industry for general purpose interconnect characterization to
move the calibration reference plane from the coaxial connec-
tor to printed board interfaces. These methods are proven by
the industry and are applicable to printed board characteriza-
tion as well. Two of such methods are outlined in 1.3.1 and
1.3.2. However, for the accurate characterization of propaga-
tion constant of the uniform transmission line section, simpler
and more universal technique can be used as outlined in
1.2.2.
1.2.2 Eigenvalue based De-embedding Methodology for
Printed Board Trace Insertion Loss Measurement
For
printed board trace characterization, there are simple
approaches to derive the printed board insertion loss, when
the DUT is a uniform transmission line. There are multiple pub-
lications proposed that using T-matrix of an ideal transmission
line segment can significantly simplify the de-embedding algo-
rithm. The T-matrix is diagonal exponential in the modal space
when normalized to the modal characteristic impedance of the
transmission line [1]-[6]. If T-matrix of a multi-conductor line
segment is converted to S-matrix, the result is an
S-parameters (where reference impedance is defined as the
characteristic impedance of the transmission line):
S
DUT
=
[
0
e
−γ L
e
−γ L
0
]
(Eq.1)
where γ is the complex propagation constant, and L the line
length. An eigenvalue based de-embedding procedure can be
carried out utilizing the above assumptions, by measuring S
parameters of two different routing lengths. There are various
(similar) derivations procedures, and below is one example:
In Figure 1-3, two printed board conductors with different
lengths (L1 and L2) are fabricated on the same test coupon.
If we pick the mid-point of L1 structure, and use T-matrices to
describe the network parameter of left and right portion of the
structure as T
A
and T
B
, then we have
T
L1
= T
A
x T
B
(Eq. 2)
T
L2
= T
A
x T
DUT
x T
B
(Eq. 3)
where DUT is the transmission line with length of L2-L1. From
(1) and (2) we can easily get
T
L2
x T
L1
-1
= T
A
x T
DUT
x T
B
x T
B
-1
x T
A
-1
= T
A
x T
DUT
x T
A
-1
(Eq. 4)
Therefore, T
L2
x T
L1
-1
and T
DUT
are similar matrices and should
have the same eigenvalue. Meanwhile, assuming the DUT is a
uniform transmission line, we have:
T
DUT
=
[
e
γ (L2-L1)
0
0
e
−γ (L2-L1)
]
(Eq.5)
Where γ is the complex propagation constant of the trans-
mission line. There are two eigenvalues of the matrix
T
L2
x T
L1
-1
(the two non-zero diagonal terms in equation 4),
where the one with absolute value <1 is the printed board
conductor loss corresponding to the routing length of (L2-L1).
Once the eigenvalue is identified, the insertion loss is readily
IPC-25514-1-3
Number
2.5.5.14
Subject
Measuring High Frequency Signal Loss and Propagation on
Printed Boards with Frequency Domain Methods
Date
02/2021
Revision
IPC-TM-650
Figure
1-3
Two-line
Structure
for
Eigenvalue-based
Method
Page
2
of
11
available based on equation (1). Note that the de-embedded
insertion loss is defined with a reference impedance of the
transmission line.
1.3 General Calibration/de-embedding Methods to Set
up Correct Reference Plane for Printed Board Conduc-
tor Insertion Loss Characterization
As mentioned earlier,
there are existing calibration/de-embedding methods for gen-
eral purpose interconnect characterization to move the cali-
bration reference plane to printed board interfaces. These
methods are validated by the industry, and therefore included
herein, although they are either more complicated or costly
than the Eigen-value based method.
1.3.1 TRL Calibration
The TRL (and its variants such as
LRM) method [7] is a general approach to move the calibra-
tion reference plane from the coaxial connector to printed
board interfaces. Figure 1-4 shows the typical calibration
structures for a TRL calibration, with microwave probe foot-
print (with single-ended probing as an example). The TRL cali-
bration technique only relies on the characteristic impedance
of the transmission line and does NOT need the parasitics of
Reflective Standard to be known, nor propagation delay of
Line. A typical TRL calibration structure may also include a
Load structure that works only at very low frequencies, and
additional Line structures to cover a wide frequency range.
Most VNAs offer TRL calibration options, please refer to the
manual or application note for your specific equipment to per-
form a TRL calibration.
TRL calibration has been widely used in the industry since the
technique no longer requires accurate calibration termination
standards. This overcomes the difficulties of SOLT calibration,
and the reference plane can be moved to the printed board.
However, there are still some disadvantages to the TRL cali-
bration. For example, there are many components of the cali-
bration standard to handle. This takes substantial printed
board area and requires tedious calibration process in the lab,
while being prone to the operator error. Additionally, the TRL
technique requires accurate characteristic impedance specifi-
cation for the line standard, which is problematic to determine
in a dispersive environment.
1.3.2 2X-Thru De-embedding
In the last decade, the
2X-thru de-embedding methodology is gaining popularity due
to its simplicity of test fixture design and de-embedding pro-
cedures [8]. In contrast to the TRL calibration technique,
which requires measurement of multiple structures as shown
in Figure 1-4, 2X-Thru De-embedding requires only one
de-embedding structure.
The basic idea of the 2X-Thru de-embedding approach is
shown in Figure 1-5. The S-parameters of the 2X-thru
IPC-25514-1-4
Number
2.5.5.14
Subject
Measuring High Frequency Signal Loss and Propagation on
Printed Boards with Frequency Domain Methods
Date
02/2021
Revision
IPC-TM-650
—
Thru
Reflective
Line
1
Figure
1-4
Calibration
Structures
(with
probing
footprint)
for
a
TRL
Calibration
Example
Page
3
of
11